1. Field of the Invention
The present invention relates to a process for producing an optically active 2,5-diamino-1,6-diphenyl-3-hexanone derivative, and more particularly to a process for producing an intermediate for a compound represented by the formula (9): EQU A--X--B (9)
wherein X means the formula (10): ##STR3## in which R.sup.1 ' is a hydrogen atom or a lower alkyl group, and R.sup.2 ' and R.sup.3 ' are, independently, an arylalkyl group or a cycloalkylalkyl group, and A and B denote R.sup.6 '--C(O)--(NH)--(CH(R.sup.5 '))--(CO)-- and R.sup.6 '--C(O)--, in which R.sup.6 ' is R.sup.7 '--NH--, R.sup.7 '--N(lower alkyl)-- [R.sup.7 ' being a (heterocyclic ring)alkyl group], and R.sup.5 ' is a lower alkyl group, said compound inhibiting retrovirus proteases, in particular, HIV proteases participating in the proliferation and expression of characters of human immunodeficiency viruses (HIV), and pharmaceutically permissible salts thereof or prodrugs (Japanese Patent Application Laid-Open No. 308574/1992).
2. Description of the Background Art
The compound represented by the plane formula (11): ##STR4## which is an intermediate for a compound having an HIV protease inhibitory action has eight optical isomers attributable to the three asymmetric carbon atoms. Of these, only a substance derived from (2S,3S,5S)-2,5-diamino-1,6-diphenyl-3-hexanol represented by the formula (12): ##STR5## exhibits an anti-HIV protease activity, and those derived from other optical isomers substantially have no anti-HIV protease activity. In order to efficiently synthesize the intended compound exhibiting the anti-HIV protease activity, it is therefore necessary to establish a process for efficiently synthesizing either the compound having the (2S,3S,5S) configuration and represented by the formula (12), or an intermediate therefor.
As examples of the process for synthesizing the compound (12), have been known the process reported by Gaush et al. [J. Org. Chem., 58, 1025 (1993)], the process reported by Kemp et al. (Japanese Patent Application Laid-Open No. 308574/1992), and the like.
Besides, the compound (12) is obtained by incorporating 4 hydrogen atoms into the double bond and the ketone group in an optically active compound represented by the formula (13): ##STR6## and eliminating the protective benzyl groups therefrom. However, 4 isomers based on the asymmetric carbon atoms situated at the 3- and 5-positions are formed according to a common process. Further, there has been proposed a process in which the compound (13) is treated with excess amounts of sodium borohydride and an acid to derive a compound represented by the formula (14): ##STR7## and the compound (14) is catalytically reduced to eliminate the protective benzyl groups, thereby producing the compound (12). However, this process involves a disadvantage that it is unfit for mass production because the purity of the above product and the yield of the intended compound are low, an expensive reagent must be used in a great amount, and a great amount of waste liquor is discharged.